It was known in the art to manage heat-imparted wear to the large exit duct to prolong the lifespan using a cold skin assembled to the hot skin in a manner to form a cooling cavity therebetween which allowed to remove heat by two effects: first, the cold skin was provided with impingement holes through which cooling air passes to remove heat from the hot skin by convection; and second, the cooling air also passes through effusion holes in the hot skin, creating a protective air film on the hot surface. This effect combination was achieved using an air tight connection between the cold skin and the hot skin.
It was known to achieve the air tight connection using welding at both ends of the cold skin. However, the hot skin being exposed to higher temperature than the cold skin, its thermal growth was greater than that of the cold skin and the cold skin would be exposed to high stress which could lead to premature cracking at the weld joint. Hence the cold skin had to be relatively long so that internal stresses imparted therein by the thermal growth of the hot skin would not exceed established thresholds. This led to an extra amount of weight corresponding to the extra length, and extra weight is undesired in aircraft applications.
It was also known to achieve the connection using welding at one end, and a sliding forged ring assembly at the other end. However, this added significant costs in addition to weight.
Accordingly, there remains room for improvement in addressing the management of stress in the cold skin.